Investigations will focus on the following aspects of host-parasite relationships in EB virus infections: 1) clinical, epidemiologic, and virologic aspects of primary infections in two age groups, children and young adults; 2) identification of specific cells in which EBV replicates in oropharyngeal and other tissue sites in the respiratory tract; 3) elucidation of rheumatoid arthritis; 4) defining factors which limit proliferation of EVBA cells in vivo, and which induce extensive plasmacytic differentiation of these cells; 5) use of the spot hybridization technique for detection of viral encoded RNA, and application of the spot method for assaying for EBV in blood, saliva, and tissues.